1 /*
2 * A module for stripping a specific TCP option from TCP packets.
3 *
4 * Copyright (C) 2007 Sven Schnelle <svens@bitebene.org>
5 * Copyright © CC Computer Consultants GmbH, 2007
6 *
7 * This program is free software; you can redistribute it and/or modify
8 * it under the terms of the GNU General Public License version 2 as
9 * published by the Free Software Foundation.
10 */
11
12 #include <linux/module.h>
13 #include <linux/skbuff.h>
14 #include <linux/ip.h>
15 #include <linux/ipv6.h>
16 #include <linux/tcp.h>
17 #include <net/ipv6.h>
18 #include <net/tcp.h>
19 #include <linux/netfilter/x_tables.h>
20 #include <linux/netfilter/xt_TCPOPTSTRIP.h>
21
optlen(const u_int8_t * opt,unsigned int offset)22 static inline unsigned int optlen(const u_int8_t *opt, unsigned int offset)
23 {
24 /* Beware zero-length options: make finite progress */
25 if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
26 return 1;
27 else
28 return opt[offset+1];
29 }
30
31 static unsigned int
tcpoptstrip_mangle_packet(struct sk_buff * skb,const struct xt_tcpoptstrip_target_info * info,unsigned int tcphoff,unsigned int minlen)32 tcpoptstrip_mangle_packet(struct sk_buff *skb,
33 const struct xt_tcpoptstrip_target_info *info,
34 unsigned int tcphoff, unsigned int minlen)
35 {
36 unsigned int optl, i, j;
37 struct tcphdr *tcph;
38 u_int16_t n, o;
39 u_int8_t *opt;
40
41 if (!skb_make_writable(skb, skb->len))
42 return NF_DROP;
43
44 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
45 opt = (u_int8_t *)tcph;
46
47 /*
48 * Walk through all TCP options - if we find some option to remove,
49 * set all octets to %TCPOPT_NOP and adjust checksum.
50 */
51 for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
52 optl = optlen(opt, i);
53
54 if (i + optl > tcp_hdrlen(skb))
55 break;
56
57 if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
58 continue;
59
60 for (j = 0; j < optl; ++j) {
61 o = opt[i+j];
62 n = TCPOPT_NOP;
63 if ((i + j) % 2 == 0) {
64 o <<= 8;
65 n <<= 8;
66 }
67 inet_proto_csum_replace2(&tcph->check, skb, htons(o),
68 htons(n), 0);
69 }
70 memset(opt + i, TCPOPT_NOP, optl);
71 }
72
73 return XT_CONTINUE;
74 }
75
76 static unsigned int
tcpoptstrip_tg4(struct sk_buff * skb,const struct xt_action_param * par)77 tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
78 {
79 return tcpoptstrip_mangle_packet(skb, par->targinfo, ip_hdrlen(skb),
80 sizeof(struct iphdr) + sizeof(struct tcphdr));
81 }
82
83 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
84 static unsigned int
tcpoptstrip_tg6(struct sk_buff * skb,const struct xt_action_param * par)85 tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_action_param *par)
86 {
87 struct ipv6hdr *ipv6h = ipv6_hdr(skb);
88 int tcphoff;
89 u_int8_t nexthdr;
90 __be16 frag_off;
91
92 nexthdr = ipv6h->nexthdr;
93 tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
94 if (tcphoff < 0)
95 return NF_DROP;
96
97 return tcpoptstrip_mangle_packet(skb, par->targinfo, tcphoff,
98 sizeof(*ipv6h) + sizeof(struct tcphdr));
99 }
100 #endif
101
102 static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
103 {
104 .name = "TCPOPTSTRIP",
105 .family = NFPROTO_IPV4,
106 .table = "mangle",
107 .proto = IPPROTO_TCP,
108 .target = tcpoptstrip_tg4,
109 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
110 .me = THIS_MODULE,
111 },
112 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
113 {
114 .name = "TCPOPTSTRIP",
115 .family = NFPROTO_IPV6,
116 .table = "mangle",
117 .proto = IPPROTO_TCP,
118 .target = tcpoptstrip_tg6,
119 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
120 .me = THIS_MODULE,
121 },
122 #endif
123 };
124
tcpoptstrip_tg_init(void)125 static int __init tcpoptstrip_tg_init(void)
126 {
127 return xt_register_targets(tcpoptstrip_tg_reg,
128 ARRAY_SIZE(tcpoptstrip_tg_reg));
129 }
130
tcpoptstrip_tg_exit(void)131 static void __exit tcpoptstrip_tg_exit(void)
132 {
133 xt_unregister_targets(tcpoptstrip_tg_reg,
134 ARRAY_SIZE(tcpoptstrip_tg_reg));
135 }
136
137 module_init(tcpoptstrip_tg_init);
138 module_exit(tcpoptstrip_tg_exit);
139 MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
140 MODULE_DESCRIPTION("Xtables: TCP option stripping");
141 MODULE_LICENSE("GPL");
142 MODULE_ALIAS("ipt_TCPOPTSTRIP");
143 MODULE_ALIAS("ip6t_TCPOPTSTRIP");
144